1. Field of the Invention
The present invention relates to an electromagnetic actuator for attracting and releasing a movable member to and from a core by selectively energizing and deenergizing a coil.
2. Description of the Related Art
One known electromagnetic actuator is used in an actuating unit for a fuel injection valve that is mounted in a cylinder head for injecting fuel into a combustion chamber of an internal combustion engine.
The known electromagnetic actuator has an electromagnet comprising a coil wound around a bobbin and a core inserted in the bobbin and forming a magnetic path. The electromagnetic actuator also includes a movable member that has an outside diameter equal to the outside diameter of the core. When the coil is selectively energized and de-energized, the movable member can be attracted to and released from a distal end of the core for moving a valve body coupled to the movable body to inject fuel into the combustion chamber.
In order to achieve accurate fuel injection timing, it is desirable to increase the response of the movable member to attractive forces generated by the electromagnet. In addition, for injecting the fuel under a relatively high pressure to promote the atomization of the fuel, the high fuel pressure tends to develop a resistance to the opening and closing movement of the valve body, failing to make the movable body sufficiently responsive to the electromagnet""s attractive forces. For this reason, the electromagnet is required to produce sufficiently large attractive forces.
The electromagnet can produce sufficiently large attractive forces if the core and the movable member are large in size. However, the core and the movable member that are large in size make it difficult to provide a necessary space in which to install the electromagnet on the internal combustion engine, and are unduly heavy. The heavy movable member is liable to make itself less responsive than desirable.
It is therefore an object of the present invention to provide an electromagnetic actuator which is capable of generating sufficient attractive forces, has a movable member highly responsive to generated attractive forces, and can be made compact.
To achieve the above object, there is provided in accordance with the present invention an electromagnetic actuator comprising a core combined with a coil, a movable member disposed so as to be attractable to an end face of the core, the movable member having an abutting surface for abutment against the end face of the core, and means for selectively energizing and de-energizing the coil to attract the movable member to and release the movable member from the end face of the core, the end face of the core being greater in size than the abutting surface of the movable member.
Since the end face of the core is greater in size than the abutting surface of the movable member, a sufficiently large magnetic path is provided to reduce a magnetic resistance for producing greater magnetic attractive forces for attracting the movable member. Because the abutting surface of the movable member is smaller in size than the end face of the core, the movable member can be reduced in size and weight for an increased response to attractive forces by which it is attracted to the core.
The end face of the core has an attracting surface for attracting the abutting surface of the movable member, and a tapered surface progressively reduced in diameter toward the attracting surface. The tapered surface of the end face of the core is effective to increase a flux density at the attracting surface for thereby concentrating the attractive forces on the abutting surface of the movable member. The movable member can thus reliably and quickly be attracted to the core for an increased response.
The tapered surface is preferably inclined from a line perpendicular to an axis of the core toward the axis of the core at an angle in the range from 40xc2x0 to 60xc2x0 or neighboring degrees. If the angle at which the tapered surface is inclined (hereinafter referred to as xe2x80x9ctaper anglexe2x80x9d) were 0xc2x0, then the tapered surface would not be formed and would blend flatwise into the attracting surface. If the taper angle were 90xc2x0, then the tapered surface would not be formed and the end face of the core would comprise the attracting surface only, so that the outside diameter of the core would be equal to the outside diameter of the movable member.
When the taper angle is smaller than 40xc2x0, the magnetic fluxes are led along the outer surface of the movable member and suffer an increased loss, resulting in a reduction in the flux density at the end face of the core, so that the attractive forces are reduced. When the taper angle is greater than 60xc2x0, the magnetic path of the core is narrowed and the magnetic resistance of the core is increased, resulting in a reduction in the flux density at the attracting surface of the core, so that the attractive forces are reduced. Sufficient attractive forces can be generated if the taper angle is in the range from 40xc2x0 to 60xc2x0. Inasmuch as attractive forces are not sharply reduced even if the taper angle falls slightly out of the range from 40xc2x0 to 60xc2x0, sufficiently high attractive forces can be produced if the taper angle is in the neighborhood of the range from 40xc2x0 to 60xc2x0.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.